Electrical apparatus



D. A. BELL ET AL ELECTRICAL APPARATUS Filed March 20, 1942 June 8, 1943.

2 Sheets-Sheet l j Fla B r 1 a6 /7 B1 c nirol FM'oui'puf 7" jafrcgFrequency Pqwer Mulfiplier Amplifier IN VEN TORS ATTORNEY June 8, 1943.D. A. BEL E AL 2,321,354

ELECTRICAL APPARATUS Filed March '20, 1942 2 Shegts-Sheet 2 Phase shfting networks INVENTORS E amid (LI/0644490665 9 MJMWM ATTORNEY PatentedJune 8, 1943 ELECTRICAL arraaarns David Arthur Bell and Owen Hosmer'Davie, London, England, assignors to Radio Patents Corporation, acorporation of New York Application March 20, 1942, Serial No. 435,541In Great Britain March 25, 1941 10 Claims. (c1. 250-36) The presentinvention relates to vacuum tube oscillators and has for its main objectthe provision of an oscillator the output frequency of which may beadjusted or controlled in an easy and efiicient manner by means of asource of control potential.

Another object of the invention is to provide a single tube frequencymodulated oscillator which will supply frequency modulated oscillationsof substantially constant amplitude.

A furtherobject is the provision of a simplified, single tube frequencymodulated oscillator suitable for producing wide frequency swingswithout distortion as required in wide band frequency modulatedtransmitters and servicing os cillators.

The above and further objects of the invention will become more apparentfrom the following detailed description taken with reference to theacompanying drawings forming part ofthis specification and wherein:Figures 1 and 2 of the accompanying drawings are circuit diagrams of S-cillators embodying the principles of the invention each employing asingle valve; Figure 3 represents, partly in blocked diagram form, acomplete frequency modulated transmitter embodying an improvedoscillator according to the'invention; Figure 4 is a circuit diagram ofanother oscillator embodying the invention and employing a single valve;Figures 5 and 6 are circuitdiagrams of further oscillators embodying theinvention each employing two valves.-

Like reference numerals identify like parts in the several figures ofthe drawings. V

A feedback oscillator comprises essentially (a) a resonant circuit and(b) a circulation path including an amplifier, an input circuit to theamplifier from the resonant circuit, and a feedback path from the outputof the amplifier to the resonant circuit. If the oscillator is tomaintain oscillations of a frequency which is displaced from the trueresonant frequency determined by the inductance and capacitance of theresonant circuit, it is necessary that the phase angle of thecirculation path shall compensate for the phase angle exhibited by theresonant circuit at that frequency.

The present invention contemplates a feed back oscillator having aplurality of circulation paths of different electrical characteristics,each arranged to feed back regenerative energy, and means to vary theratio of the energies fed back through said paths in accordance with anapplied bias or modulating potential.

In apreferred arrangement, partsof said.'cirphase angles.

In order to maintain reasonable constancy of amplitude when thefrequency is varied by varying the applied potential, the said phaseangles should be so selected that the sum of the purely regenerativecomponents of the energies fed back in the whole of the circulationpaths will remain fairly constant. If there are two circulation paths,this implies that the rates of change with modulating potential of thepurely regenerative components of the energies fed back in the two pathsshall be substantially equal and opposite.

In one method of carrying the invention into eifect, separate thermionicvalves are included in the different circulation paths and. the ratio ofthe energies fed back through said paths is varied by variation ofcontrol potentials applied to said thermionic valves.

In another arrangement for carrying the invention into effect,asingle'valve of the pentode or similar type is employed, By a valve ofpentode or similar type in this specification is meant as those of theSAC] type commercially available,

the output electrodes may be the screen grid and the anode while themodulation grid may be the suppressor grid. In a hexode, such as the 6K8type, the modulation'grid may be-the third grid while the outputelectrodes are the coupled sec- 0nd and fourth grids on the one hand andthe anode on the other hand.

When a pentode or'similar type of valve'is employed, one of thecirculation. paths is arranged to include the feedback circuit. of oneoutput electrode and another of the circulation paths is arranged toinclude thefeedback circuit have the same phase angle, provided thatthisis not zero; and as one form of this a single net- ()FFICE work mayform a feedback circuit common to both output electrodes.

The anode current of a thermionic valve depends jointly on the anodevoltage and the grid voltage, the relative importance of these twofactors depending upon the amplification factor. If the amplificationfactor is very large, the anode voltage has little effect, and the anodecurrent is then substantially in phase with the grid voltage regardlessof the nature of the anode circuit. If, however, the amplificationfactor is sufficiently small for the anode voltage to have appreciableeffect, then the phase of the anode current will depend upon the phaseangle of the anode load.

If the anode circuit contains reactance, the phase of the anode currentwill then differ appreciably from that of the grid voltage.

In a valve of pentode or similar type, the amplification factor betweenthe first output electrode (screen grid) and the control grid whichcontrols the cathode current (inner control grid) is relativelyilow;while the amplification factor between the second output electrode(anode) and that control grid is very high. It follows that, if thefeedback networks have the same phase angle, or if a single feedbacknetwork is employed in common, the phase angle of the total feedbackcurrent relative to the inner control voltage will be dependent upon thedistribution of current between the two output electrodes, and will thusbe dependent upon the control potential applied to the modulationcontrol grid.

In order to maintain reasonable constancy of amplitude with change ofamplification factor, it is necessary to arrange that the purelyregenerative component of the total energy fed back shall not varysubstantially.

The distribution of current betweenthe two output electrodes isinfluenced, not only by the potentialof the modulation control grid, butalso by the potentials of the output electrodes themselves. By properchoice of the impedances of the output circuits, this effect can be madeto secure substantial constancy of output for all potentials of themodulation control grid within the modulation range, .in spite of thedifferent characteristics of the valve with respect to the outputs fromthe two electrodes. 1

Alternatively, the feedback networks shall include different amountsofattenuation so that, althoughthe anode voltages are unequal, theregenerative energies delivered to the tuned circuits are substantiallyequal.

In Figure 1 .of the accompanying drawings, there isshown a pentode I I]provided in the mannercknown with a cathode II, an inner control gridI2, a screen grid I3, a suppressor or outer control grid I4, and ananode or plate I5, all arranged substantially in the order named withrespect to said cathode. An oscillatory tank circuit comprising, in theexample shown, an inductance coil I shunted by a variable condenser I1is coupled for high frequency currents to both the screen grid I3 andplate I5 used as the output electrodes through a coupling condenserThere are further provided suitable impedances such as high ohmicresistors 20 and 2I arranged in the anode andscreen grid circuits,respectively, and these circuits are further parallelled and connectedthrough a common impedance 22 to the positive terminal of a suitablesource of space current supply such as a battery or the output of arectifier indicated in a customary manner by the plus symbol in thedrawing. Feedback to the control grid I2 from the oscillatory circuitI5, I! 75 20 and 2|,Figure 1, being omitted. The necessary is effectedby Way of a coupling inductance 23 arranged in inductive relation withthe tank circuit inductance I6 and coupled to the control grid andcathode through grid coupling condenser 24. The control grid is providedwith a grid leak 25 in the manner and for the purpose well understood bythose skilled in the art. Any other known type of regenerative circuitarrangement may be employed for the maintenance of sustainedoscillations in the circuit IS, I! as will become obvious from thefollowing:

The .outer control or suppressor grid I4 is biased through a suitablemodulation source connected to terminals a, b by means of a fixedpotential sufficiently negative (see Figure 3) to enable the tube towork on the linear portion of the suppressor grid characteristic and toprevent the potential on the suppressor grid from rising above cathodepotential.

The frequency at which the circuit aforedescribed will oscillate isdependent on the potential of the suppressor grid and bears a linearrelationship to the latter over a considerable band of operatingfrequencies. The amplitudes of the screen grid oscillatory current andof the anode oscillatory current are also each dependent upon thesuppressor grid potential, but it is possible to arrange that the sum oftheir purely regenerative components will be substantially independ entof the suppressor grid potential by suitable choice of the impedances20, 2|, and 22. Amplitude variations of the oscillatory voltage acrossthe circuit I6, II are thus substantially eliminated, while retainingdependence of the frequency of oscillation upon the suppressor gridpotential. Thus, by coupling a control or modulation source .of anysuitable wave form to terminals a, 17 such as for example a sine wave orsaw-tooth oscillator, or an audio frequency voltage, the frequency ofthe generated oscillations may be. modulated in a corresponding manner.

The variation of the oscillating frequency may be due, not only to theeffects above described but also in part to a change of thecathode-toplate capacity of the tube which is effectively shunted acrossthe oscillatory circuit IE, II, the capacity variations being in turnthe result of the'changing space charge distribution in thecathode-anode space caused by potential changes on the outer controlgrid I In order to avoid disturbance of the parameters of theoscillatory circuit, the output of the tube is preferably derived bymeans of a tuned circuit of low Q comprising an inductance 26 shunted bya condenser 21 and being in coupling relation to the tank circuitinductance I6.

If the amplitude of the oscillatory voltage developedacross the tankcircuit I6, I! is found to be not quite sufiiciently constant when thefrequency is modulated, compensation of the amplitude variations may beeffected by tuning the output circuit 26, 21 to a frequency at the endof a frequency modulation band at which the amplitude is low.

The tuning of the tank circuit may be varied over a range by varying thecapacitance of the condenser I'I.

Figure 2 shows a circuit diagram of a modified oscillator circuitaccording to the invention utilizing a tube 28 of the triode-hexodetype. In this embodiment, the anode I5 is directly coupled throughcondenser 29 to the second and fourth grids forming the screen I3enclosing the outer control grid I4, the impedances corresponding torelative adjustment of the currents in the output circuit in this caseis obtained by the adjustment of series impedances and 3| connected inthe leads to the space current supply source of the screen grid andplate electrodes, respectively.

In order to obtain a wide frequency modulation band it is necessary thatthe tuned circuit I6, I I shall be fairly heavily damped. If theimpedance of the network 29, 30, 3| is too high for this purpose anadditional resistance (not shown) may be connected in parallel with theoscillatory circuit I8, II.

The circuit according to Figure 2 is otherwise similar to that shown inFigure 1 but has certain additional features. The hexodes, at presentcommercially available such as the popular 6K8 type, are nearly alltriode-hexodes, and in the exemplification of Figure 2 which shows atube of this type the triode section comprising the common cathode II,the control grid I2, directly internally connected to the outer controlgrid I4 of the hexode section, and the plate I5, is employed to providenegative feedback which has been found to widen the band of linearmodulation obtainable. This is effected by including a resistance 32 inthe cathode lead of the tube shunted by a condenser 33 of such capacityas to offer low impedance to currents of the oscillatory frequencygenerated and high impedance to the current of the modulation frequencyor band of modulation frequencies impressed upon the triode control gridI2. Alternatively, the triode section of the tube may be connected in aseparate feedback oscillator circuit to generate an oscillatory voltageof any desired frequency for modulating the frequency of the oscillatorycircuit connected to the 'hexode section.

If an exceptionally wide frequency modulation band is required, this maybe obtained by heterodyning the outputs of two oscillators of thecharacter hereinbefore described, in which case the outer control gridsof these oscillators are modulated in phase opposition. If atriode-hexode is employed in one of these oscillators, its triodesection may be used to provide the necessary change of the phase of themodulation voltage for application to the valve of the other oscillator.

Referring to Figure 3, there is shown a complete frequency modulatedtransmitter embodying a master oscillator of the type proposed by theinvention. The oscillator circuit shown is similar to that according toFigure 1 with the exception that the outer control grid I4 is excited bythe secondary or an audio frequency transformer 35, the primary of whichis connected in a modulating circuit including a current source 36 and amicrophone 31 in series. The source which may be a battery or any othersuitable voltage supply, also serves to bias the grid I4 to a potentialsufliciently negative with respect to the oathode for the purposepointed out above. The frequency modulated output voltage developedacross the tank circuit I6, I? is impressed upon a suitable poweramplifier 38, if desirable, by way of a frequency multiplier 39 toincrease both the frequency and the power sulnciently for energizing theantenna 40. The same system may be used for producing a phase modulatedoutput by the provision of a suitable corrective network or integratingcircuit such as a series resistance and shunting condenser insertedbetween the microphone and outer control grid circuits in a manner wellunderstood by those skilled in the art. Furthermore, means formaintaining constant or stablizing the centre frequency may be providedin standard frequency (crystal) oscillator beating with oscillationsderived from the power ampliher or antenna circuit to produce a controlfrequency which is impressed upon a frequency discriminator producing acontrol voltage varying in sign and magnitude in accordance with thedeparture of the oscillator centre frequency from its assigned value;This control voltage which may be due to oscillator drifts and othercauses is applied to the outer control grid I 4 together with themodulation voltage in such phase as to counteract any initial centrefrequency deviation in a manner well known in the art.

The oscillator shown in Figure 4 is shown as comprising a single heirodevalve I0 having like elements as the hexode portion of the valve inFigure 2. r

The tuned circuit consists of inductance 44 in parallel with condenser45 and is connected in the control grid circuit of the valve.

The high tension supplies to anode and screen grid are made throughseparate resistors 30 andv 3| of high value.

In the feed back path from the anode a blocking condenser 43 isintroduced, but the phase angle of this path is substantially determinedby the inductance 4|. The phase angle of the feed back path from thescreen grid is substantially determined by the condenser 42, which may.have a reactance at oscillation frequency substantially equal inmagnitude to that of inductance 4|. The feed back is effected throughinductance 46, common to both feed back paths, coupled with inductance44 of the tuned circuit. The output voltage developed between terminalsc and d is derived through coupling condenser 49 directly from thevoltage appearing across the tuned circuit. Its frequency is modulatedby varying the potential applied between terminals or and b to themodulation control grid and thereby varying the distribution of currentbetween screen grid and anode of the valve. Bias for both control gridsis produced in conven-- tional manner by resistance 41 shunted bycondenser 48 which exhibits low reactance at both oscillation andmodulation frequencies.

In Figure 5, two triode oscillator valves 10 and 'II are provided, andthe tuned circuit compris ing inductance 44 and condenser 45 isconnected in common in the grid circuits of these valves.

Separate feed back circuits from the anodes are connected throughblocking condensers 43, I3

and phase shifting networks 16, TI to a common coupling coil 46 which iscoupled to inductance 44 of the tuned circuit. The ratio of the energiesfed back in the two paths is controlled by vary ing the relative anodepotentials of valves 10 and 'II. The high tension source for theseanodes is connected to the 'midpoint of the secondary winding 62 of atransformer, the primary winding SI of which is connected between theterminals 0. b of the modulation source. The anodes of the valves 10, IIare connected to the opposite extremities of secondary winding 62through choke coils I4, I5 respectively which offer high impedance tocurrents of oscillation frequency. 7

The output voltage appearing between ter- .minals c and d is derivedthrough coupling conaccordance with known practice comprising a Thephase shifting networks 15 and H can produce any desired angles of phaseshift, 'provided that these angles are different and that the feed backthrough each is regenerative. Pref erably these phase angles shouldbe'of equal magnitudeand opposite sign. If desired, however, one. ofthese networks may be omitted while the other may, for example, have aphase angle of about.45. Over the extreme modulation range,

as the feed back is substantially wholly trans ferred from one valve tothe other, there will then be a change in the proportion of the inphasecomponent of the voltage fed back of the order of \/2 to 1, assuming theattenuation in both feed back networks to be the same. The actual changein amplitude of the oscillation generated will be less than this owingto the curvature of the valve characteristics, and for many purposes anamplitude variation of this order will not be objectionable.

In Figure 6 the tuned circuit comprising inductance H3 in parallel withcondenser I1 is connected in common in the anode circuits of pentodevalves 53, 54 between the anodes and the high tension source. Separatecoils 58 and 60 are coupled to inductance IE to supply separate feedback paths to the control grids to the two pentode valves. Between coil59 and the control grid of valve 53 a substantially 45 lagging phaseshift is introduced by the network comprising resistance 5'! andcondenser 58; the reactance of condenser 58 being substantially equal atoscillation frequency to the value of resistance 51. Similarly asubstantially 45 leading phase shifting is introduced between coil 60and the control grid of valve 54 by a network comprising condenser 56and resistance 55, the reactance of condenser 56 at oscillationfrequency. being substantially equal to the value of resistance 55. Thescreen grids of both pentode valves are maintained at constantpotential.The energies handled by the valves are varied oppositely by connectingsuppressor grids to opposite ends of the secondary winding 62 of thetransformer, the primary winding 6| of which is connected to theterminals a b of the modulation voltage source. The centre point of thesecondary winding 62 is maintained at earth potential. Bias for bothcontrol grids of each valve is obtained in conventional manner byresistance 41 shunted by condenser 48, which exhibits low reactance atboth modulation and oscillation frequencies. The output" voltagedeveloped between terminals 0 and d is derived through couplingcondenser 49 from the anodes of the valves; its frequency is modulatedin accordance with the voltage variations of the modulation sourceapplied between terminals w and b.

Itwill be evident from the foregoing that the invention is not limitedto the specific details and circuits shown herein for illustration, butthat the underlying basic principle is susceptible of variations, withinthe broader scope and spirit of the invention as defined in the appendedclaims. The specification and drawings are, accordingly, to be regardedin an illustrative rather than in a limiting sense.

We claim:

1. In an oscillator, an electron discharge tube comprising a cathode, afirst control grid, a screen grid forming a first output electrode, asecond control grid and a second output electrode all arrangedsubstantially in the order named, an oscillatory circuit operativelyconnected to said first grid and to both said output electrodessubstantially in parallel relation for the oscillating output currentsto generate sustained electrical oscillations, the non-common circuitportions from said output electrodes to said oscillatory.

2. In an oscillator, an electron discharge tube comprising a cathode, afirst control grid, a screen grid forming a first output electrode, asecond control grid and a second output electrode all arrangedsubstantially in the order named. an oscillatory circuit operativelyconnected to said first grid and to both said output electrodessubstantially in parallel relation for the oscillating output currentsto generate sustained electrical oscillations, impedance means in thenon-common connecting leads from said oscillatory circuit to said outputelectrodes to adjust the normal ratio of the oscillating currentcomponents supplied by said output electrodes to substantiallycompensate amplitude variations of the total oscillating current in saidcircuit in response to potential variations on said second grid, saidimpedance means having substantially the same phase anglecharacteristics, and a source of variable control potential connected tosaid second control grid and cathode for varying the frequency of theoscillations produced.

3. In an oscillator, an electron discharge tube comprising a cathode, afirst control grid, a screen grid forming a first output electrode, asecond control grid and a second output electrode all arrangedsubstantially in the order named, a tuned oscillatory circuit coupled toboth said output electrodes and said cathode in substantially (parallelrelation for the oscillating output currents, a further couplingconnection between said oscillatory circuit and said first grid togenerate sustained electrical oscillations in said'circuit, impedancemeans in the non-common circuit portions to said output electrodes toadjust the normal ratio of the oscillating current components suppliedby said output electrodes to substantially compensate amplitudevariations ofthe total oscillating current in said circuit in responseto control potential variations of said second grid, said impedancemeans having substantially the same phase angle characteristics, and asource of variable control potential connected to said second grid andcathode for varying the frequency of the oscillations produced.

4. In an oscillator, an electron discharge tube comprising a cathode, afirst control grid, a screen grid forming a first output electrode, asecond control grid and a second output electrode all arrangedsubstantially in the order named, a tuned oscillatory circuit connectedto both said output electrodes in substantially parallel relation forthe oscillating output currents, further coupling means between saidoscillatory circuit and said first grid to generate sustained electricaloscillations in said circuit, impedance means in the noncommon circuitportions to said output electrodes from said oscillatory circuit toadjust the normal ratio of the oscillating current components suppliedby said output electrodes to substantially compensate amplitudevariations of the total oscillating current in said circuit in responseto control potential variations'of said second grid,

means for applying a steady negative bias potential to said secondcontrol grid of such magnitude as to prevent said control grid potentialfrom rising above the cathode potential, the non-common circuit portionsbetween said oscillatory circuit and said output electrodes havingsubstantially the same phase characteristics, and a source of variablecontrol potential connected to said second grid and cathode for varyingthe frequency of the oscillations produced.

5. In an oscillator, an electron discharge tube comprising a cathode, acontrol grid and a pair of output electrodes, means to provide arelatively low amplification factor between said control grid and one ofsaid output electrodes and to provide a relatively high amplificationfactor between said control grid and the other output electrode, anoscillatory circuit operatively connected to said grid and to both saidoutput electrodes substantially in parallel relation for the oscillatingoutput currents to generate sustained electrical oscillations, thenon-common circuit portions from said output electrodes to saidoscillatory circuit having substantially the same phase anglecharacteristics, and means to control the relative oscillating currentsupplied by said output electrodes to correspondingly vary the frequencyof the oscillations produced.

6. In an oscillator, an electron discharge tube comprising a cathode, acontrol grid, and a pair of output electrodes, means to provide arelatively low amplification factor between said control grid and one ofsaid output electrodes and to provide a relatively high amplificationfactor between said control grid and the other output electrode, anoscillatory circuit operatively connected to said first grid and to bothsaid output electrodes substantially in parallel relation for theoscillating output currents to generate sustained electricaloscillations, the non-common circuit portions from said outputelectrodes to said oscillatory circuit having substantially the samephase angle characteristics, and a current distribution control gridbetween said output electrodes for controlling the ratio of theoscillating currents supplied by said output electrodes to said circuitto correspondingly vary the frequency of the oscillations produced.

7. In an oscillator, an electron discharge tube comprising a cathode, acontrol grid, a screen grid forming a first output electrode and asecond output electrode all arranged substantially in the order named,an oscillatory circuit operatively connected to said control grid and toboth said output electrodes substantially in parallel relation for theoscillating output currents to generate sustained electricaloscillations, the noncommon portions from said output electrodes to saidoscillatory circuit having substantially the same phase anglecharacteristics, and means to control the relative oscillating currentsupplied by said output electrodes to said circuit to correspondinglyvary the frequency of the oscillations produced.

8. In an oscillator, an electron discharge tube comprising a cathode, afirst control grid, a screen grid forming a first output electrode, asecond control grid and a second output electrode all arrangedsubstantially in the order named, an oscillatory circuit operativelyconnected to said first grid and to both of said output electrodessubstantially in parallel relation for the oscillating output currentsto generate sustained electrical oscillations, ohmic resistance means inthe non-common connecting leads from said output electrodes to saidoscillatory circuit to adjust the normal ratio of the oscillatingcurrent components supplied by said output electrodes, and means toapply variable control potential of said second grid to correspondinglyvary the frequency of the oscillations produced.

9. In an oscillator, an oscillatory circuit, electron space dischargemeans comprising input means and a pair of output electrodes, circuitmeans between said oscillatory circuit to said input means, furthercircuit means operatively connecting said oscillatory circuit to bothsaid output electrodes substantially in parallel relation for theoscillating output currents to generate sustained electricaloscillations, means to provide a relatively high amplification factorbetween said input means and one of said output electrodes and toprovide a relatively low amplification factor between said input meansand the other output electrode, the non-common circuit portions fromsaid output electrodes to said oscillatory circuit having substantiallythe same phase angle characteristics, and means to control the relativeoutput current supplied by said output electrodes to said circuit tocorrespondingly vary the frequency of the oscillations produced.

10. In an oscillator, an oscillatory circuit, electron space chargemeans comprising input means and a pair of output electrodes, circuitconnections between said oscillatory circuit and said input means,further connections between said oscillatory circuit and both saidoutput electrodes substantially in parallel relation for the oscillatingoutput currents to generate sustained electrical oscillations in saidcircuit, means to provide a relatively high amplification factor betweensaid input means and one of said output electrodes and to provide arelatively low amplification factor between said input means and theother output electrode, non-reactive impedance means inserted in thenon-common connecting leads from said output electrodes to saidoscillatory circuit to adjust the normal ratio of the output currentcomponents, and means to control the ratio of the output currentcomponents in respect to said normal ratio to correspondingly vary thefrequency of the oscillations produced.

OWEN HOSMER DAVIE. DAVID ARTHUR BELL.

